AQP4 and neuromyelitis optica: Previous studies have revealed consistently greater affinity of NMO-IgG binding to M23 than M1 AQP4 (10), and Fab fragments of NMO rAbs showed a similar preference of AQP4 isoform binding, indicating that structural changes in AQP4 upon array assembly, and not bivalent cross-linking of whole IgG, result in the greater binding affinity to OAPs.